Who goes first

In a first or single game the players pick the starting player by any fair method, such as a die roll. In a match, the loser of the previous game chooses.

The rule

For the first game of a match, or a single game, the players decide who chooses the starting player by any method they agree on, a coin flip or a die roll (CR 103.1). Turns then go clockwise from the starting player.

In a match of several games the loser of the previous game chooses who goes first. If that game was a draw, the player who chose last time chooses again (CR 103.1). A game is a draw when all remaining players lose at the same time (CR 104.4a).

Formats with shared team turns, such as Two-Headed Giant, pick a starting team instead of a starting player (CR 103.1a). In Archenemy there is no pick: the archenemy always takes the first turn (CR 103.1b).

In Life Counter

Tap to roll for the first turn and the app draws one seat at random, every seat with the same chance. In Two-Headed Giant it draws a team.

On a rematch between two sides the app hands the choice to the loser of the last game, and after a draw to whoever chose before, as the rule says. A table of three or more rolls again.